The investment casting process typically uses a refractory mold that is constructed by the buildup of successive layers of ceramic particles bonded with an inorganic binder around an expendable pattern material such as wax, plastic and the like. The finished refractory mold is usually formed as a shell mold around a fugitive (expendable and removable) pattern. The refractory shell mold is made thick and strong enough to withstand: 1) the stresses of steam autoclave or flash fire pattern elimination, 2) the passage through a burnout oven, 3) the withstanding of thermal and metallostatic pressures during the casting of molten metal, and 4) the physical handling involved between these processing steps. Building a shell mold of this strength usually requires at least 5 coats of refractory slurry and refractory stucco resulting in a mold wall typically 4 to 10 mm thick thus requiring a substantial amount of refractory material. The layers also require a long time for the binders to dry and harden thus resulting in a slow process with considerable work in process inventory.
The bonded refractory shell molds are typically loaded into a batch or continuous oven heated by combustion of gas or oil and heated to a temperature of 1600° F. to 2000° F. The refractory shell molds are heated by radiation and conduction to the outside surface of the shell mold. Typically less than 5% of the heat generated by the oven is absorbed by the refractory mold and greater than 95% of the heat generated by the oven is wasted by passage out through the oven exhaust system.
The heated refractory molds are removed from the oven and molten metal or alloy is cast into them. An elevated mold temperature at time of cast is desirable for the casting of high melting temperature alloys such as ferrous alloys to prevent misruns, gas entrapment, hot tear and shrinkage defects.
The trend in investment casting is to make the refractory shell mold as thin as possible to reduce the cost of the mold as described above. The use of thin shell molds has required the use of support media to prevent mold failure as described U.S. Pat. No. 5,069,271 to Chandley et al. The '271 patent discloses the use of bonded ceramic shell molds made as thin as possible such as less than 0.12 inch in thickness. Unbonded support particulate media is compacted around the thin hot refractory shell mold after it is removed from the preheating oven. The unbonded support media acts to resist the stresses applied to the shell mold during casting so as to prevent mold failure.
Thin shell molds, however, cool off more quickly than thicker molds following removal from the mold preheat oven and after surrounding the shell with support media. This fast cooling leads to lower mold temperatures at the time of casting. Low mold temperatures can contribute to defects such as misruns, shrinkage, entrapped gas and hot tears, especially in thin castings.
U.S. Pat. No. 6,889,745 to Redemske teaches a thermally efficient method for heating a gas permeable wall of a bonded refractory mold wherein the mold wall defines a mold cavity in which molten metal or alloy is cast. The mold wall is heated by the transfer of heat from hot gas flowing inside of the mold cavity to the mold wall. Hot gas is flowed from a hot gas source outside the mold through the mold cavity and gas permeable mold wall to a lower pressure region exterior of the mold to control temperature of an interior surface of the mold wall. Despite the usefulness of the mold heating process described in the '745 patent, uneven pattern elimination and uneven mold heating have been observed, where the top of the mold heats much faster than the bottom, which can result in shell cracking at the top and incomplete pattern elimination at the bottom. This may be addressed by heating the thin shell refractory molds at a slower rate in order to promote temperature uniformity, but results in very long burn-out cycles; as long as seven hours. In addition, due to initial low gas permeability as binders are burned out of the mold wall, pattern elimination can be problematic due to difficulty in starting and operating burners at the low burn rates governed by poor gas permeability, resulting in multiple restarts of the burner to establish a reliable flame. In addition, the mold heating method described in the '745 patent is useful with thin shell refractory molds that have relatively high gas permeability through the mold walls as described, but is not useful for thick shell refractory molds having relatively low gas permeability or no gas permeability.
Accordingly, it is desirable to provide refractory molds and methods of making and using the molds that are capable of maintaining uniform mold temperatures throughout the mold and that are useful for all types of refractory molds, regardless of the thickness gas permeability of the mold wall.